96 KERATIN AND KERATINIZATION 



membrane, the primary hair germ. As proliferation continues, the bud 

 projects downwards and penetrates the dermis as a solid cord of cells. 

 Perhaps conditions here favour further division, the cells finding more food 

 and living space. The advancing tip calls forth a response from the dermis 

 shown by the approach of a number of dermal cells. The two kinds of 

 cells do not mix, since they are already separated by the basement 

 membrane. The advancing epidermal cells spread over the dermal 

 cluster to form a cap, the future hair bulb, and the mesodermal elements 

 within form the future papilla. These events are partly recapitulated each 

 time a new hair develops in a resting follicle (see Fig. 40). 



This co-operative relation between dermal cells and epidermal has 

 impressed many observers; moreover, the morphogenetic control which 

 subsequently develops in the follicle is also dependent upon the continued 

 presence of the dermal component. This is shown perhaps most clearly 

 by Hardy's (1949, 1951) experiments on the formation of hair in tissue 

 culture and by equivalent experiments concerning the development of 

 feather germs (Lillie, 1942). In the absence of dermal remnants, true 

 follicles never develop in tissue culture; together, dermis and epidermis 

 can produce and maintain follicles with differentiating cells. No other 

 accessory structures, such as blood vessels or nerves, are required. 

 Moreover, the arrangement of the follicle in groups is similar to that of 

 skin growing in situ. This is an important demonstration of the morpho- 

 genetic competence of the skin and it emphasizes that the control of 

 differentiation in the epidermis is localized (see p. 146). 



After the papilla has formed within the tip of the cells descending 

 from the basal layers of the epidermis, mitosis becomes restricted to the 

 lower half of the follicular bulb. The hair now sprouts up from this 

 matrix and penetrates the originally-solid plug of epidermal cells which 

 thus become the outer root sheath (Fig. 40). The advancing tip forms a 

 cone of cells continuous with those of Henle's layer of the inner root 

 sheath. 



Typically follicular activity is cyclic, a growing phase is followed by a 

 resting phase (Montagna, 1956); Fig. 40 depicts, for example, the events 

 in the cycle of growth in rat skin. Cyclic growth is discussed in the next 

 chapter. In a condition of steady growth the hair follicle offers a remark- 

 ably compact example of organogenesis. From the mass of undifferentiated 

 cells in the lower half of the bulb (Fig. 42) arises a solid cylinder composed 

 of six concentric cylinders each consisting of cells which become different 

 in shape and which follow different paths in internal development. In 

 terms of their characteristic products of synthesis, the three external 

 cylinders, each a single layer of cells,* which comprise the inner root 



* The layer of Huxley may consist of more than one layer in thickness, in 

 asymmetrical hairs (Rudall, 1956). 



